Answer:
You can tell a guy likes you if he leans towrds you, turns his body towards you, looks at you in the eye, wants to be closer to you plus many more ways.
Edit:
All of the above is true.
Additional : We do lots of things with our arms such as stretching or flexing. We also yawn alot and move closer to the girl often (If were not to shy).
Yeah this is true. Also, if the person you're suspicious of sits in front of you at some point, he'll most likely sit sideways in his desk. Plus- all that stuff you heard that a guy will pick on you if he likes you is complete bull.